kv: add comprehensive tests for Batch operations

This commit adds extensive unit tests for the kv.Batch type, covering
all major operations including Get, Put, Scan, Delete, conditional
operations, bulk operations, and admin methods. Tests also cover
error handling, buffer management, and raw vs non-raw batch mixing.

Release notes: None

Epic: None

Author: vishalv1994

pkg/kv/BUILD.bazel

@@ -51,6 +51,7 @@ go_test(
     size = "medium",
     srcs = [
         "admission_test.go",
+        "batch_test.go",
         "client_test.go",
         "db_test.go",
         "main_test.go",